Internal combustion engines have spark plugs which are electrically connected to ignition coils which provide properly timed electric signals to the spark plugs when the engine is running. In the course of determining the cause of a particular malfunction of an engine it often becomes necessary to determine whether or not a particular spark plug is in fact being properly energized from the ignition coil. To that end it has been common to disconnect the usual electric connection between the ignition coil and the spark plug and replace it with a testing device such as an illuminable bulb which will indicate whether the spark plug is receiving the proper signals from the ignition coil and to insert that device between the ignition coil and the spark plug under test. The testing device is provided with electrical connectors at each end which may be connected to the spark plug and the ignition coil respectively and which, when the testing is completed, may be disconnected therefrom, after which the normal electrical connection between the ignition coil and the spark plug is reestablished. Typical patents showing such testing devices are Whitehead U.S. Pat. No. 6,714,015 of Mar. 30, 2004, entitled “Apparatus for Testing Spark Plugs”, Mitani, et al. U.S. Pat. No. 5,599,195 of Feb. 4, 1997, entitled “Ignition Plug Cap”, McCord U.S. Pat. No. 2,853,691 of Sep. 23, 1958, entitled “Tune-up Extension for Use With Spark Plugs for Testing Purposes”, and Korba U.S. Pat. No. 4,026,621 of May 31, 1997, entitled “Timing Light Adapter”. The aforementioned '015 patent discloses in FIGS. 1 and 2 units of the type under discussion previously sold by the assignee of this application.
Spark plugs have at their upper ends exposed male electrical contacts so that a female connector may be readily attached thereto and detached therefrom, and the testing devices of the prior art have therefore been provided with female connectors which may be pressed onto male spark plug contacts when testing is required. In the past spark plugs were mounted on the cylinder head so that their male contacts were readily accessible. However, in more modern internal combustion engines the spark plugs are housed within deep recesses in the cylinder head, which recesses are normally covered so as to prevent foreign matter from entering the recesses and adversely affecting the functioning of the spark plugs. As a result it has become quite difficult to reliably attach a testing device to the spark plug within its cylinder recess, and in some instances to prevent foreign matter from entering the recess during the testing operation. In addition, in order to ensure the maintenance of a proper electrical connection between the testing device and the spark plug it is desirable for that portion of the testing device which is received within the recess to be more or less firmly supported, and since spark plug recesses in different engines vary significantly in diameter, providing adequate support for the tester, thus ensuring sufficient support during the testing procedure, presents a very significant problem.